Metadata is data. When Shannon says "meaning", he is including control channel information as well. He is distinguishing variable messages from copying constants, which is a different and easier challenge.
Meaning in this case essentially means context. You have a communications channel, it delivers to you four 32-bit integers, what do they represent? Positional coordinates? One pixel in RGBA? In CMYK? IPv4 port/address 4-tuple with zero-padded 16-bit ports? A single IPv6 address? Big endian or little endian?
